1. Field of the Invention
The present invention relates generally to an apparatus and a method for controlling an uplink frequency in a mobile communication system. More particularly, the present invention relates to an apparatus and a method for controlling a frequency to mitigate interference between uplink signals of a user equipment in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) mobile communication system.
2. Description of the Related Art
Multimedia systems of the future need to meet demands for fast transmission of high-capacity data, and an increasing demand for wireless communication. Recently, research has been conducted relating to a transmission scheme that allows for high-speed data transmission with high frequency utilization. To transmit high-speed data of a short symbol period, a wide transmission band is required and a frequency selective fading, caused by Inter-Symbol Interference (ISI) or multi-path interference of a radio channel, should be overcome. Multi-carrier techniques transmit data using multiple carriers by extending the symbol period. An Orthogonal Frequency Division Multiplexing (OFDM) scheme, which is one of the multi-carrier techniques, features high frequency utilization by overlapping orthogonal frequency bands of the subcarriers, and fast modulation and demodulation using the multiple carriers through Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT). The OFDM scheme can achieve the simple equalization with one tap per subcarrier in the frequency domain by sustaining the orthogonality of the subcarriers using a guard interval longer than a delay spread of the signal of the multi-path channel.
An Orthogonal Frequency Division Multiple Access (OFDMA) transmission scheme sends data using multiple carriers. The multi-carrier maintains a minimum frequency spacing (subcarrier spacing) and thus utilizes the entire frequency band with efficiency.
However, the OFDMA transmission scheme, which is very sensitive to a frequency offset, suffers from Inter-Carrier Interference (ICI) caused by some frequency offset.
To mitigate the interference of the frequency offset, a general mobile communication system adopts a frequency control method using an oscillator and a frequency control method using a numerically controlled oscillator.
FIG. 1 depicts the frequency control method of a static channel in the general mobile communication system.
When a base station transmits a DownLink (DL) signal using a Radio Frequency (RF) center frequency of fRF—BS—DL in FIG. 1, the spectrum is generated as shown in (a).
Next, provided that a frequency offset ΔfRx—UE is generated in consideration of an oscillator variation between the user equipment and the base station while an antenna of the user equipment receives a receive signal in (b), the user equipment receives the receive signal over the RF center frequency fRF—UE—DL as shown in (c). When the user equipment compensates for the frequency offset using an Automatic Frequency Controller (AFC), the signal is received with the transmit and receive frequencies matched as shown in (d).
Meanwhile, UpLink (UL) data of the user equipment is transmitted using a transmit frequency fRF—UE—DL of the frequency spacing fRF—UE—DL−fRF—UE—UL as shown in (e). The base station can receive the signal compensated by the AFC of the user equipment as shown in (f) and (g).
FIG. 2 depicts the frequency control method of a mobile channel in the general mobile communication system.
When the base station transmits a DL signal using the RF center frequency fRF—BS—DL in FIG. 2, the spectrum is generated as shown in (a). Doppler frequency offset generated when the user equipment moves around causes the reception spectrum as shown in (b).
Accordingly, the difference between the RF receive frequency and the RF transmit frequency of the user equipment produces the frequency offset Δfdoppler—DL+Δfosc which combines the Doppler frequency offset and the offset of the transmit and receive frequencies as shown in (c).
Next, when the AFC of the user equipment compensates the frequency offset, the user equipment receives the receive signal of (d).
Meanwhile, the data transmission from the user equipment to the base station uses a transmit frequency fRF—UE—UL with the frequency spacing fRF—UE—DL−fRF—UE—UL as shown in (e). The signal received at the base station suffers the Doppler frequency Δfdoppler—DL as shown in (f).
When the user equipment and the base station use the multi-path channel under the same condition, the Doppler frequency of the base station faces the same direction as the Doppler frequency of the user equipment as shown in (f).
This problem arises when the user equipment moves at a low speed, rather than at a high speed. Hence, a receiver of the base station is subject to the additional frequency offset.
FIG. 3 depicts the signal received at the base station in the general mobile communication system.
When a plurality of user equipments accesses the base station at the same time, the user equipments suffer from the Doppler frequency according to their different channel environments. The signals of the fast moving user equipments and the signals of the slow moving user equipments of FIG. 3 cause the ICI in the uplink of the multi-access scheme, such as OFDMA, and thus degrade the reception performance.